1. Field of the Invention
The present invention relates to sound systems, and more particularly to sound systems which provide a sense of three-dimensional depth in listening experience.
2. Prior Art
The evolution of sound reproduction began with a simple sound source such as a horn loudspeaker acoustically coupled to a rotating cylinder which carried physical impressions of sound scribed into its surface. The emitted sound was very localized, propagating from the horn with a directional aspect oriented along the horn throat axis. As speakers became more sophisticated, stereophonic features were added in combination with use of multiple speaker systems, generating left and right or side-to-side dynamics to sound reproduction. Modem surround sound systems capitalize on diverse speakers to generate both stereophonic and multichannel output, as well as synchronized shifting of isolated sounds to individual speakers disposed around the listener. In this manner, for example, sound associated with motion picture display can develop greater realism by coordinating specific events on the screen with shifting sound propagation around the room from a variety of directions.
Because of the physiology of the ear, human hearing is amazingly capable of assigning a directional aspect to sound. This ability provides a continuous flow of information to the brain, supplying data which is assimilated in defining an individual's position and environment within a three-dimensional framework. Modem surround sound systems simulate a desired three-dimensional environment by directing sound to the listener from various orientations, including front, side, back, floor and ceiling propagation. Such sounds include speaking voices from persons at differing positions, surrounding environmental sounds of nature such as water movement, wind, thunder, birds, animals, etc. Action scenes include synthesized audio effects for emphasizing mood dynamics of anxiety, fear, surprise, and pleasure, as well as sound effects for crash scenes, explosions, and a myriad of visual objects whose display on the screen is brought to life with multidirectional sound effects.
In order to implement effective surround sound experience as described above, conventional sound systems include many speakers, positioned around a room perimeter, including floor and ceiling. Typically, low range woofers are located at the front of the room, or under the floor. Because these low frequency speakers have less directionality, their location at the front of a room is not problematic. Indeed, the low range sound is difficult to ascribe to any direction when the room is reasonably small in dimension. Because of the large size of conventional dynamic speakers, location in the front of the room is generally more practical.
With increasing frequency, directional aspect of sound propagation is enhanced. Tweeters, for example, can readily be detected as to source or orientation. Surround sound systems supply these higher frequencies from smaller speakers which are dispersed at the sides and back of the room, enabling their beaming properties to simulate sound emanating from multiple directions as if in a natural environment. Physical displacement and positioning at wall and ceiling locations is facilitated by the smaller size of this speaker component.
Parametric speakers are also known for their highly directional character. U.S. Pat. No. 4,823,908 of Tanaka et. al. discloses that the derivation of audio output from a modulated ultrasonic carrier provides a more focused directivity, even at lower frequency ranges. FIG. 2 of this patent shows a conventional parametric system 8 oriented directly toward a listener 9. Acoustic filters 10 and 20 or other acoustic absorbing material are therefore applied along the audio path between the emitter and listener for substantially eliminating the ultrasonic component of the parametric output. Although reflective plates 19 are disclosed in Tanaka et. al. (i.e. FIG. 16), their sole purpose appears to be for lengthening the acoustic path and changing the direction of propagation of the ultrasonic and audio frequencies. Accordingly, prior art teachings with respect to parametric speakers do not distinguish any significant difference between audio output between direct projection of parametric output toward a listener and indirect propagation of such audio output to a listener by reflection, except perhaps enhanced db level.
In accordance with this understanding, prior art systems for developing perception of sound sources from different directions would necessitate the placement of a speaker along a particular orientation and at a predetermined location. In order to obtain multiple directions as part of a surround sound experience, multiple speakers (dynamic, electrostatic, parametric, etc.) at differing locations would be required.
Therefore, the need to disperse speaker systems at a variety of positions within the listener's experience generally necessitates more complex technical implementation. Speaker wires must extend from sound source to speaker hardware. For in-home theaters, retrofit of wiring may be expensive and/or detrimental to room decor. Efforts to avoid unsightly wiring may include FM wireless transmission systems which are very expensive and often problematic in operation. Even where new construction allows prewiring of surround sound systems, limited adaptability exists because the speakers are fixed at certain locations, and are not subject to rapid relocation consonant with displacement of the sound. If a sense of movement is desired based on shifting sound source, many speakers are required along the direction of movement, with complex crossover circuitry to synchronize sound through the desired speaker devices. In short, the excessive cost and complexity of dynamic movement of the sound source has discouraged general commercial application in environments other than public move theaters.